This invention relates to a termination system for use with optical fibres, and in particular with optical fibres which have been installed in a tubular passageway using a flow of air or other gas. A method of installing an optical fibre in this way is known, for example, from EP-A-0108590. EP-A-0108590 uses an air flow which is blown along this tubular passageway, and for convenience the method is referred to below as blowing, though it should be mentioned that it has been proposed elsewhere to use sucking instead of, or in addition to, blowing, and the system of the present invention is equally applicable if sucking is used.
A common way of attaching modern communications equipment to incoming and outgoing cables at network hubs (e.g. equipment rooms), is via patch leads and a patch panel. These patch facilities are similar in concept to old fashioned operator controlled telephone exchanges. A patch arrangement provides organised and easily traceable terminations. The connections can also be reconfigured, so affording a simple method of line switching. This is particularly suitable for high bandwidth systems, wherein other switching techniques may prove unreliable and expensive.
A popular way of housing communication devices, such as opto-electronic line drivers, at network hubs, is via rack style equipment cabinets. Patch panels for use with metallic and optical cables have been developed for use in equipment cabinets. These are particularly favoured, because they allow standard cabinet sizes to be used for cable terminations and communications equipment. This assists and simplifies room layout.
Simple optical patch panels are essentially the same as those intended for use with metallic cables. A typical known optical patch panel is shown in FIG. 1 and comprises an array of optical connector coupling sleeves 100 mounted on a support plate 101. The panel can be easily incorporated into network cabinets, via the vertical support rails of the cabinet. The design enables optical cables 102 containing a single optical fibre and each terminated with a male optical connector 103, to be linked by the coupling sleeve. FIG. 2 shows a side view of such an arrangement.
Simple optical patch panels have been produced for a variety of different types of optical connector. They have also been produced for twin fibre cables terminated with a duplex optical connector.
The use of coupling sleeves to align two male connectors, rather than male/female pairs, results from the need to polish the end of the fibre during the termination operation. This ensures a good optical connection through the coupling. Female connectors would prove awkward to polish and are not normally produced.
Patch panels are often used to terminate and patch multifibre cables. With the possible exception of twin fibre cables, multifibre varieties generally require a modified patch panel design. This is to allow organisation of the fibres contained within the cable which assists termination. A further requirement of patch panels used for terminating and patching multifibre cables, is to allow the storage of coils of excess fibre. These excess lengths allow several termination attempts on each fibre within the cable. This avoids the need to interfere with fibres already terminated with optical connectors, for example in the event of a fibre breaking when attaching a connector. Without the coils of excess fibre this would require more of the cable sheath to be removed to reveal more of the broken fibre. This could result in damage to those fibres already terminated.
Patch panels intended for use with multifibre cables provide a means of fibre storage and organisation on one side of the connector coupler support panel. The other side is unchanged and allows attachment of single or perhaps twin fibre patch leads or multi-way patch leads. These are used to link the terminated fibres of the multifibre cable to optoelectronic equipment in the usual way. FIG. 3a shows a simple termination and patching module 105 for use with multifibre cables. This comprises a casing 106 having a base 107 on which the fibre is coiled and a front plate 108 on which there are coupling sleeves 109.
Termination of fibres with optical connectors, involves stripping the outer protective coatings of the fibre, gluing the exposed glass into a connector ferrule (an accurately bored metal or ceramic tube), assembling the remainder of the connector around the ferrule and finally polishing the tip of the fibre and ferrule to produce a transparent fibre/air boundary. Optical connectors are often attached to the fibres at installation sites. Field termination, as this technique is termed, is messy, time consuming and can result in poor terminations. This is due to dusty and dirty installation environments often encountered at installation sites.
An alternative is to splice connector pigtails to the cabled fibres, for example by fusion welding techniques. Fusion welding involves melting the free ends of the pigtails, each of which comprises an optical connector pre-terminated onto a short length of fibre, to the cabled fibre, using one of the many portable proprietary fusion welding machines available. This technique has the advantage of allowing the messy connector attachment and polishing to be carried out in a controlled factory environment. This contributes to the provision of consistent and reliable terminations. However, if this technique is used additional storage space is required for the pig-tailed fibre. Termination and patch modules for multifibre cables are ideally suited to termination of fibres using the pigtail technique.
FIG. 3b shows the module 105 used with optical fibres having pigtails 111. The fusion splice protectors 110 shown in the FIG. 3 each comprise a short heat shrinkable tube containing a metal pin. This affords protection to the mechanically weak splice.
Modifications which can be made to the basic termination and patch module, include the provision of individual fibre storage cassettes for the coils and excess fibre and fibre pigtails. These improve organisation of the fibres within the module and are particularly useful when terminating cables containing many fibres
With conventional optical cables, once the fibres have been terminated within a module, there is generally no requirement to gain access to the fibre storage area. One possible reason for access would be to repair a damaged fibre. However such damage is extremely unlikely as the fibres are well protected. Termination modules are therefore often installed in a rigid stack, the terminations in each module being completed and the module securely fastened before the next module is put into place above. This approach makes future access to the storage compartment difficult in the unlikely event of a repair being necessary but does produce a compact termination cabinet.
Such inaccessible terminations would be useless for blown fibre systems, since after optical fibres have been installed in tubular passageways by blowing, it may be necessary at a later time to regain access to the ends of the tubular passageways, which are typically in the form of flexible plastics tubes. One reason for requiring access is that initially the system may have spare tubes, i.e. more tubes than are required to carry the number of optical fibres which are initially needed. At a later date, as and when additional fibres are needed, these may be installed in the spare tubes, and this requires access to the ends of the tubes. Secondly, it may sometimes be necessary to remove fibres from tubes, for example to allow replacement fibres to be installed, and removal, like installation, is carried out by blowing. This too, requires access to the ends of the tubes.
Our European Patent Application published as EP-A-0408266 describes a termination system which permits the access referred to above, without needing to interfere with the operation of other optical fibres connected through the termination system. To this end, the termination employs modules each of which slides between a first position in which the module is enclosed in the housing and a second position in which the module extends from the housing to allow access to the interior of the module, each module being provided with means for terminating an end of at least one passageway in which an optical fibre may be installed and means for terminating an end of at least one further optical fibre member.
However, although the termination system described in EP-A-0408266 has advantages compared to the prior art, the sliding mechanism it employs makes it quite expensive to manufacture. Also, it introduces an element of potential unreliability, because the blowing tubes have to move when the modules are moved, which could damage the tube.